The present invention relates to a signal modulating device, and more particular to a continuous-time sigma-delta modulator.
For a continuous-time sigma-delta modulator (CT-SDM), the feed-forward topologies are often chosen over feedback topologies because the feed-forward topologies has lower internal signal swings and fewer feedback digital-to-analog elements. However, the feed-forward topologies exhibit large out-of-band peaking in signal transfer function (STF), which can be a problem in wireless applications having strong interferers/blockers. More specifically, the out-of-band peaking can reduce the amount of filtering provided by baseband or intermediate-frequency filters when out-of-band interferers are presented and result in degraded dynamic range of the modulator. Meanwhile, larger out-of-band signals can potentially overload the input of quantizer and lead to an unstable modulator. Therefore, how to reduce the out-of-band signals of a continuous-time sigma-delta modulator is an urgent problem in the field of wireless communications.